Does CloudStack allow upload of Data Volumes to clusters which only have local storage enabled?

Not at this time (As of CloudStack 4.1.x). See CLOUDSTACK-4173 for the enhancement request

Can CloudStack Advanced Networking be created without using VLANs?

VLANs are the default isolation method for CloudStack Advanced Networks. Other than VLAN isolation, GRE isolation and STT isolation is also supported by CloudStack in an Advanced Zone. Other SDN technologies like Nicira is also supported

Is it possible to use CloudStack with VMWare ESXi Hypervisors without using vCenter Server?

No. vCenter server is a key requirement for CloudStack to talk to ESXi Hypervisors

Can Domain Admins create new users to the domain they are administering?

On CloudStack 4.2, domain admins cannot add new users to their administered domain. See CLOUDSTACK-4830